1. Field of the Invention
This invention relates to radiotelephone transmitter/receivers and in particular, but not exclusively it relates to transmitter tracking loops for radiotelephones.
2. Description of the Related Art
Reference is made to the following papers for technological background material relating to the invention:
xe2x80x9cRecent Advances in RF Integrated Circuitsxe2x80x9d by Behzad Razavi published in IEEE Communications Magazine December 1997.
xe2x80x9cOversampling and sigma-delta strategies for data conversionxe2x80x9d by R. W. Stewart and E. Pfann published in Electronics and Communication Engineering Journal February 1998.
xe2x80x9cAn Agile ISM (Industrial, Scientific and Medical use) Band Frequency Synthesizer with Built-In GMSK Modulationxe2x80x9d by Norman M. Filiol, Thomas A. D. Riley, Calvin Plett and Miles A. Copeland published in IEEE Journal of Solid-State circuits, Vol 33, no. 7 July 1998.
UK patent application GB 2 320 629 A.
It is now a common practice to make use of a tracking loop architecture for mobile phones for operation with GSM (Global System for Mobile communications, formerly Groupe Special Mobile). This tracking loop architecture uses a Voltage controlled oscillator (VCO) operating at the required transmission frequency to provide the output carrier frequency i.e. an xe2x80x9con-channelxe2x80x9d transmitter. The transmitter VCO is phase locked and within the loop bandwidth of this VCO the modulated data passes. Through the process of down conversion and phase detection, the modulation on the output of the VCO is-compared with the original (desired) modulation. The negative feedback thereby implemented provides for accurate reproduction of the modulation at radio frequency.
Fabrication of this type of system requires a significant area of RF silicon for the down converter, high frequency phase detector and quadrature modulator. Owing to the low yield and the quantity of wafers used for implementation of circuits in RF Silicon, costs for RF Silicon are relatively high when compared with standard CMOS circuitry. For this reason it is desirable to implement such a modulation loop but using only CMOS technology and so allowing for integration with the base band circuitry.
It is therefore an object of the present invention to provide a radiotelephone transmitter/receiver reducing its cost and size.
According to the invention a part of the output signal from the voltage control oscillator is down-converted by the down-converting means. The down-converted signal is compared with a modulation signal by the phase detector. The signal has amplitude and polarity based on the comparison effected between the down-converted signal and the modulation signal is provided to the voltage control oscillator. The voltage control oscillator outputs a signal has frequency based on the applied signal.